<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .duct_up{
            background: url("img/up_mod.png");
            background-repeat: repeat-y;
            height: 120px;
            position: relative;
        }
        .duct_up img{
            position: absolute;
            bottom: 0;
        }
        .duct_down{
            background: url("img/down_mod.png");
            background-repeat: repeat-y;
            height: 200px;
            position: absolute;
            bottom: 0;
            width: 100%;
        }
        .duct_down img{
            position: absolute;
            top: 0;
        }
    </style>
</head>
<body>
    <div class="duct_up">
        <img src="img/up_pipe.png">
    </div>
    <div class="duct_down">
        <img src="img/down_pipe.png">
    </div>
</body>
<script>
    // setInterval 循环定时器
    // setInterval(function(x,y){
    //     var x = "软帝";
    //     var y = "666";
    //     document.writeln(x);
    //     document.writeln(y);
    // },2000)
</script>
</html>